1. Field of the Invention
The present invention relates to a jet-propulsion personal watercraft (PWC) which ejects water rearward and planes on a water surface as the resulting reaction. More particularly, the present invention relates to a personal watercraft which has an output shaft end portion of an engine with a unique structure.
2. Description of the Related Art
In recent years, so-called jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is configured to have a water jet pump that pressurizes and accelerates water sucked from a water intake generally provided on a hull bottom surface and ejects it rearward from an outlet port. Thereby, the personal watercraft is propelled.
In the jet-propulsion personal watercraft, a steering nozzle provided behind the outlet port of the water jet pump is swung either to the right or to the left, to change the ejection direction of the water to the right or to the left, thereby turning the watercraft to the right or to the left.
In the personal watercraft, a two-cycle engine has been conventionally employed as a propulsion engine. In addition to this, the use of a four-cycle engine has been contemplated.
In order to obtain a power as high as that of the two-cycle engine in the four-cycle engine without an increase in displacement, increasing an engine speed is effective. However, increasing the engine speed is unfavorable in view of the compatibility with a pump characteristic of the water jet pump.
Since the personal watercraft are utilized for various purposes as described above, there are demands for diverse engines including a normal power engine for family, a high-power engine for sport activities, etc. These engines are economically achieved by changing intake and exhaust efficiency or the like of one type of engine. In that case, the compatibility with the pump characteristic of the water jet pump needs to be considered because the engine speed increases as the power increases. In particular, in the case of the personal watercraft for sport activities which perform high-speed and active cruising, it is necessary to set its maximum engine speed high. Therefore, the compatibility with the pump characteristic needs to be carefully considered.
In order to obtain a higher power in the four-cycle engine, it is necessary to increase the diameter of a crankshaft for the purpose of ensuring sufficient physical strength of the crankshaft. With an increase in the diameter of the crankshaft, it becomes necessary to increase the diameter of a crankshaft-side sprocket provided on the crankshaft for driving a camshaft, and correspondingly increase the diameter of a camshaft-side sprocket provided on the camshaft (The rotation speed of the crankshaft is reduced by half and transmitted to the camshaft-side sprocket). This increases a volume in a cylinder head and a cylinder head cover for accommodating the camshaft-side sprocket, which results in a large-sized engine. Such a large engine is difficult to mount in the personal watercraft having an engine room limited in space.
As a solution to this, metal having high strength such as chromium-molybdenum steel could possibly be used for the crankshaft for the purpose of reducing the shaft diameter, which leads to increased cost.
The present invention addresses the above-described condition, and an object of the present invention is to provide a personal watercraft capable of obtaining a high power engine without the necessity of increasing a diameter of a crankshaft-side sprocket for driving a camshaft. Another object of the present invention is to provide a personal watercraft capable of transmitting power to a water jet pump at the rotation speed lower than the rotation speed of the crankshaft of the engine while maintaining the rotation speed of the crankshaft when obtaining the high power engine.
According to the present invention, there is provided a personal watercraft comprising: a water jet pump that ejects water rearward to propel the watercraft as a reaction of the ejecting water; an engine for driving the water jet pump; and an extended shaft placed coaxially with a crankshaft of the engine and having one end portion connected to one end portion of the crankshaft, wherein a mounting hole is formed at the one end portion of the crankshaft so as to be coaxial with the crankshaft, and the one end portion of the extended shaft constitutes an insertion portion inserted into the mounting hole.
In accordance with the invention, since the extended shaft independent of the crankshaft is connected to the one end portion of the crankshaft, only the diameter of the extended shaft can be reduced, or only the extended shaft can be made of the material having high strength. So, the engine mounted in the personal watercraft can be compactly configured. It should be appreciated that the extended shaft may be connected to an output-side end portion of the crankshaft (rear-side end portion of the watercraft), or to an end portion of the crankshaft located on the opposite side of the output-side end portion (front-side end portion of the watercraft).
Preferably, the extended shaft may be connected to the crankshaft so as to be integrally rotatable.
Preferably, the extended shaft may have a diameter smaller than a diameter of the one end portion of the crankshaft.
Preferably, the extended shaft having the small diameter may be provided with a sprocket for driving a camshaft of the engine.
In the above structure, since the crankshaft-side sprocket for driving the camshaft is provided on the extended shaft having the small diameter, the diameter of the crankshaft-side sprocket and hence the diameter of the corresponding camshaft-side sprocket can be reduced. Therefore, the cylinder head can be compactly configured. Even when it becomes necessary to increase the diameter of the crankshaft in order to obtain the high power in the engine, the size of the camshaft-side sprocket need not be increased, and consequently, the size of the entire engine is not increased. In this case, a diameter of a portion of the extended shaft, which is located on the opposite side of the crankshaft with respect to the crankshaft-side sprocket, is not necessarily small.
Preferably, the mounting hole of the crankshaft may be connected to the insertion portion of the extended shaft through a bearing such that the crankshaft and the extended shaft are rotatable relative to each other. With such a structure, for example, a reduction gear is capable of transmitting the power of the crankshaft to the extended shaft at the rotation speed lower than that of the crankshaft.
The personal watercraft constituted such that the crankshaft is connected to the extended shaft through the bearing as described above, may further comprise: a parallel shaft parallel to the crankshaft and the extended shaft; a first pair of gears having gears provided on the crankshaft and the parallel shaft, respectively, for causing the crankshaft and the parallel shaft to operate in association with each other; and a second pair of gears having gears provided on the extended shaft and the parallel shaft, respectively, for causing the extended shaft and the parallel shaft to operate in association with each other, and the first pair of gears and the second pair of gears are adapted to transmit a power of the crankshaft to the extended shaft.
With such a structure, the power for rotating the crankshaft can be transmitted to the extended shaft at the rotation speed lower than that of the crankshaft.
Preferably, in the structure in which the personal watercraft comprises the parallel shaft, the first pair of gears, and the second pair of gears, the extended shaft may be provided with a sprocket for driving a camshaft of the engine.
Preferably, the first pair of gears and the second pair of gears may be adapted to transmit the power of the crankshaft to the extended shaft at a rotation speed different from a rotation speed of the crankshaft.
Preferably, the personal watercraft further comprises: a gear casing separable from a crankcase of the engine and covering the first pair of gears and the second pair of gears. With such a structure, a gear unit comprised of the first pair of gears, the second pair of gears, and the like can be easily removably attached.
Preferably, an oil reservoir may be provided in an inner bottom portion of the gear casing. This makes it possible that the first pair of gears and the second pair of gears can be lubricated by simple splashing.
Preferably, at least a wall portion of the oil reservoir in a wall portion of the gear casing may be formed to have a double-walled structure, thereby forming a water jacket. With such a structure, the gear unit where the temperature is elevated during operation of the engine can be efficiently cooled.
The above and further objects and features of the invention will more fully be apparent from the foregoing detailed description with accompanying drawings.